SEQUENCE STRATIGRAPHY AND SEISMIC STRATIGRAPHY OF THE LOWER CRETACEOUS SECTION IN THE NORTHEASTERN GULF OF MEXICO

The Lower Cretaceous (LK) section offshore Alabama and Mississippi was a petroleum target in the 1970's with oil shows in the Main Pass (MP) area, on the shelf-margin trend. From the late 1990's to the present there has been fair production on the middle shelf in the Viosca Knoll (VK) area. To date, no comprehensive analysis of this section in these locations has been published.

This work focuses on the characterization of the sequence stratigraphy, seismic stratigraphy, and the paleoenviromental distribution of the LK strata offshore Alabama and Mississippi. About 3,500 kilometers of 2D seismic reflection data shot in the study area were interpreted and integrated with GR, SP, lithology logs, and lithostratigraphic picks from more than fifty wells, and 120 meters of core from the MP area. Seven check-shot surveys were used to integrate seismic and well data.

In the study area the LK section is composed mostly of carbonates with minor siliciclastic rocks that were deposited in inner-middle shelf, shelf margin, slope, and basin environments. One well log signature type was associated with middle shelf deposits and a different one with shelf margin sediments. Associations of seismic facies ranging from parallel-continuous to chaotic helped to ascertain the depositional settings.

Eight major third-order depositional sequences were recognized. They are constrained between two regional unconformities. The youngest, the mid-Cretaceous Unconformity (MCU), indicates a deposition hiatus of 20 million years in the MP 253#6 well. Highstand systems tract and lowstand systems tract deposits are often the only strata seismically resolvable. The shelf margin, characterized by discontinuous skeletal reefs in the carbonate sequences, shifted basinward six kilometers during the LK. The shelf margin of the younger sequences underwent greater subsidence with respect to the back-reef and middle shelf areas. Tectonic activity was restricted to the shelf margin and slope settings.